1. Field of the Invention
The present invention relates to a gain control circuit suitable for, for example, a power amplifier of a mobile phone.
2. Description of the Related Art
A conventional gain control circuit shown in FIG. 3 has a differential circuit, which is an amplifying circuit having an emitter connected to the ground, incorporated thereinto. The amplifying circuit having the emitter connected to the ground includes a transistor 1, a variable resistor element 5, which is a PIN diode, and a resistor 7. Of a current flowing through an emitter of the transistor, a DC bias current is differentially branched to transistors 2 and 3. The emitter current depends on a transistor 4. An input signal is amplified by the transistor 1. Since the branch of signals at the emitter of the transistor 1 is prevented by a transistor 6, the gain is determined by the ratio of the resistor 7 on the collector side to the AC resistance of the variable resistor element 5.
The AC resistance of the variable resistor element 5 is changed in inverse proportion to a DC bias current flowing through the variable resistor element 5. Therefore, it is possible to change the gain of the transistor 1 with the DC bias current flowing through the variable resistor element 5.
The DC bias current flowing through the variable resistor element 5 is determined by a different between a control voltage applied to the base of the transistor 2 and the voltage of the base of the transistor 3 set by resistors 10 and 11. Even when the DC bias current flowing through the variable resistor element 5 is varied by the control voltage, the DC bias current flowing through the transistor 1, which is the sum of currents flowing through the transistors 2 and 3, is kept constant by resistors 12, 13, and 14 connected to the transistor 4.
According to the convention structure, it is possible to change the gain by varying the control voltage.
However, in this case, although the control voltage is constant, a variation in ambient temperature causes the DC bias current flowing through the variable resistor element 5 to be changed, resulting in a variation in the gain of the transistor 1.